Planetary gear train of automatic transmission for vehicles

ABSTRACT

A planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving torque, an output shaft disposed in parallel with and apart from the input shaft, a planetary gear set including a first sun gear, a second sun gear, a common planet, and a common ring gear engaged, a first rotation shaft directly connected to the first sun gear and selectively connected to the input shaft, a second rotation shaft directly connected to the common planet carrier and directly connected to the output shaft, a third rotation shaft directly connected to the common ring gear and selectively connected to the input shaft or to a transmission housing, and a fourth rotation shaft directly connected to the second sun gear and selectively connected to the input shaft through first and second paths or to the transmission housing.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2013-0145219 filed Nov. 27, 2013, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that can improve power delivery performance and reduce fuel consumption at a low-speed driving.

2. Description of Related Art

Typically, multiple-shift mechanism of an automatic transmission is achieved by combining a plurality of planetary gear sets and a plurality of frictional elements. It is well known that when a planetary gear train realizes a greater number of shift speeds, speed ratios of the planetary gear train can be more optimally designed, and therefore a vehicle can have economical fuel mileage and better performance. For that reason, the planetary gear train that is able to realize more shift speeds is under continuous investigation.

Though achieving the same number of speeds, the planetary gear train has a different operating mechanism according to a connection between rotation elements (i.e., sun gear, planet carrier, and ring gear). In addition, the planetary gear train has different features such as durability, power delivery efficiency, and size depending on the layout thereof. Therefore, designs for a combining structure of a gear train are also under continuous investigation.

If the number of shift-speeds, however, increases, the number of components in the automatic transmission also increases. Therefore, mountability, cost, weight and power delivery efficiency may be deteriorated.

Particularly, since the planetary gear train having a number of components is hard to be mounted in a front wheel drive vehicle, researches for minimizing the number of components have been developed.

The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing a planetary gear train of an automatic transmission for a vehicle having advantages of achieving eight forward speeds and one reverse speed by disposing a compound planetary gear set on an output shaft and connecting rotation elements of the planetary gear set to an input shaft through a plurality of transfer gears.

In addition various aspects of the present invention are directed to providing a planetary gear train of an automatic transmission for a vehicle having further advantages of enabling of setting optimum gear ratios due to ease of changing gear ratios by using the plurality of transfer gears, and accordingly improving power delivery performance and fuel economy at a low-speed driving.

In an aspect of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving torque, an output shaft disposed in parallel with and apart from the input shaft, a planetary gear set including a first sun gear engaged with a long pinion, a second sun gear engaged with a short pinion, a common planet carrier rotatably supporting the short pinion and the long pinion, and a common ring gear engaged with the long pinion as rotation elements thereof, a first rotation shaft directly connected to the first sun gear and selectively connected to the input shaft, a second rotation shaft directly connected to the common planet carrier and directly connected to the output shaft, a third rotation shaft directly connected to the common ring gear and selectively connected to the input shaft or to a transmission housing, and a fourth rotation shaft directly connected to the second sun gear and selectively connected to the input shaft through first and second paths or to the transmission housing.

The planetary gear train may further include three transfer gears disposed respectively between the input shaft and the first and third rotation shafts and on the first and second paths.

The planetary gear train may further include a dog clutch selectively transmitting torque of the input shaft to any one of the first or second paths or not transmitting torque to either of the first or second paths.

The three transfer gears may include a first transfer gear disposed on the first path; a second transfer gear disposed on the second path; and a third transfer gear disposed between the input shaft and the first and third rotation shafts.

Gear ratios of the three transfer gears may be different from each other.

The planetary gear train may further include a first clutch disposed between the third transfer gear and the first rotation shaft, a second clutch disposed between the third transfer gear and the third rotation shaft, a first brake disposed between the third rotation shaft and the transmission housing, and a second brake disposed between the fourth rotation shaft and the transmission housing.

The first and second transfer gears and the first and second brakes may be disposed at one side of the planetary gear set, and the first and second clutches and the third transfer gear may be disposed at the other side of the planetary gear set.

A planetary gear train of an automatic transmission for a vehicle according to another exemplary embodiment of the present invention may include an input shaft receiving torque; an output shaft disposed in parallel with and apart from the input shaft, a planetary gear set including a first sun gear engaged with a long pinion, a second sun gear engaged with a short pinion, a common planet carrier rotatably supporting the short pinion and the long pinion, and a common ring gear engaged with the long pinion as rotation elements thereof, a first rotation shaft directly connected to the first sun gear and selectively connected to the input shaft, a second rotation shaft directly connected to the common planet carrier and directly connected to the output shaft, a third rotation shaft directly connected to the common ring gear and selectively connected to the input shaft or to a transmission housing, a fourth rotation shaft directly connected to the second sun gear and selectively connected to the input shaft through first and second paths or to the transmission housing, and a dog clutch selectively connecting the input shaft to the fourth rotation shaft through the first and second paths.

The planetary gear train may further include a first transfer gear disposed on the first path, a second transfer gear disposed on the second path, and a third transfer gear disposed between the input shaft and the first and third rotation shafts.

Gear ratios of the three transfer gears may be different from each other.

The planetary gear train may further include a first clutch disposed between the third transfer gear and the first rotation shaft, a second clutch disposed between the third transfer gear and the third rotation shaft, a first brake disposed between the third rotation shaft and the transmission housing, and a second brake disposed between the fourth rotation shaft and the transmission housing.

The first and second transfer gears and the first and second brakes may be disposed at one side of the planetary gear set, and the first and second clutches and the third transfer gear may be disposed at the other side of the planetary gear set.

The dog clutch may be adapted to selectively transmit torque of the input shaft to any one of the two paths or not to transmit either of the two paths.

Other aspects and preferred embodiments of the invention are discussed infra.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according to an exemplary embodiment of the present invention.

FIG. 2A is a drawing for illustrating operation of a dog clutch used in a planetary gear train according to an exemplary embodiment of the present invention.

FIG. 2B is a drawing for illustrating operation of a dog clutch used in a planetary gear train according to an exemplary embodiment of the present invention.

FIG. 2C is a drawing for illustrating operation of a dog clutch used in a planetary gear train according to an exemplary embodiment of the present invention.

FIG. 3 is an operational chart of friction elements at each shift-speed applied to a planetary gear train according to an exemplary embodiment of the present invention.

FIG. 4 is a lever diagram of a planetary gear train according to an exemplary embodiment of the present invention.

Reference numerals set forth in the Drawings include reference to the following elements as further discussed below:

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

Hereinafter, an exemplary embodiment of the present invention will be described with reference to the accompanying drawings so that those skilled in the Field of the Invention to which the present invention pertains may carry out the exemplary embodiment.

Description of components that are not necessary for explaining the present exemplary embodiment will be omitted, and the same constituent elements are denoted by the same reference numerals in this specification.

In the detailed description, ordinal numbers are used for distinguishing constituent elements having the same terms, and have no specific meanings.

FIG. 1 is a schematic diagram of a planetary gear train according to an exemplary embodiment of the present invention.

Referring to FIG. 1, a planetary gear train according to an exemplary embodiment of the present invention includes an input shaft IS receiving torque of an engine, an output shaft OS disposed in parallel with and apart from the input shaft IS, a planetary gear set PG disposed on the output shaft OS, three transfer gears TF1, TF2, and TF3, dog clutch DC, and frictional elements including two clutches C1 and C2 and two brakes B1 and B2.

The input shaft IS is an input member and receives torque from a crankshaft of the engine.

The output shaft OS drives a driving shaft including a driving wheel through a final reduction gear and a differential apparatus.

The planetary gear set PG is disposed on the output shaft OS. The planetary gear set PG is a compound planetary gear of Ravingneaux type formed by combining a single pinion planetary gear set and a double pinion planetary gear set and having a planet carrier and a ring gear in common, and has four rotation shafts N1, N2, N3, and N4.

That is, the planetary gear set PG includes a first sun gear S1 engaged with a long pinion P1, a second sun gear S2 engaged with a short pinion P2, a common planet carrier PC12 rotatably supporting the long pinion P1 and the short pinion P2, and a common ring gear R12 engaged with the long pinion P1 as rotation elements thereof.

The first rotation shaft N1 is directly connected to the first sun gear S1 and is selectively connected to the input shaft IS.

The second rotation shaft N2 is directly connected to the common planet carrier PC12 and is directly connected to the output shaft OS so as to be always operated as an output element.

The third rotation shaft N3 is directly connected to the common ring gear R12 and is selectively connected to the input shaft IS or to a transmission housing H.

The fourth rotation shaft N4 is directly connected to the second sun gear S2 and is selectively connected to the input shaft IS through two paths or to the transmission housing H.

The first, second, and third transfer gears TF1, TF2, and TF3 respectively have first, second, and third transfer drive gears TF1 a, TF2 a, and TF3 a and first, second, and third transfer driven gears TF1 b, TF2 b, and TF3 b externally meshed with each other.

In addition, gear ratios of the first, second, and third transfer gears TF1, TF2, and TF3 may be set according to target speed ratios. For example, the gear ratio of the second transfer gear TF2 is smaller than that of the first transfer gear TF1 and is greater than that of the third transfer gear TF3.

Connections of the first, second, and third transfer gears TF1, TF2, and TF3 will be described in detail.

The first transfer gear TF1 connects the fourth rotation shaft N4 to the input shaft IS. That is, the first transfer drive gear TF1 a is connected to the input shaft IS and the first transfer driven gear TF1 b is connected to the fourth rotation shaft N4.

The second transfer gear TF2 connects the fourth rotation shaft N4 to the input shaft IS. That is, the second transfer drive gear TF2 a is connected to the input shaft IS and the second transfer driven gear TF2 b is connected to the fourth rotation shaft N4.

The third transfer gear TF3 connects the first rotation shaft Ni and the third rotation shaft N3 to the input shaft IS. That is, the third transfer drive gear TF3 a is connected to the input shaft IS and the third transfer driven gear TF3 b is connected to the first rotation shaft Ni and the third rotation shaft N3.

The dog clutch DC is widely used in a manual transmission and is well known to a person of an ordinary skill in the art. The dog clutch DC selectively transmits torque of the input shaft IS to the first and second transfer gears TF1 and TF2, or does not transmit the torque of the input shaft IS to either of the first and second transfer gears TF1 and TF2.

In addition, two clutches C1 and C2 connecting selected rotation shafts to the input shaft IS and two brakes B1 and B2 connecting selected rotation shafts to the transmission housing H are disposed as follows.

The first clutch C1 is disposed between the third transfer driven gear TF3 b of the third transfer gear TF3 and the first rotation shaft N1.

The second clutch C2 is disposed between the third transfer driven gear TF3 b of the third transfer gear TF3 and the third rotation shaft N3.

The first brake B1 is disposed between the third rotation shaft N3 and the transmission housing H.

The second brake B2 is disposed between the fourth rotation shaft N4 and the transmission housing H.

The frictional elements consisting of the first and second clutches C1 and C2 and the first and second brakes B1 and B2 are conventional multi-plate friction elements of wet type that are operated by hydraulic pressure.

The first and second transfer gears TF1 and TF2 and the first and second brakes B1 and B2 are disposed at one side of the planetary gear set PG, and the first and second clutches C1 and C2 and the third transfer gear TF3 are disposed at the other side of the planetary gear set PG.

FIGS. 2A-2C are drawings for illustrating operation of a dog clutch used in a planetary gear train according to an exemplary embodiment of the present invention.

Referring to FIGS. 2A-2C, the dog clutch DC includes a dog clutch hub DCH, a dog clutch sleeve DCS, and first and second dog clutch gears DCG1 and DCG2.

The dog clutch hub DCH is directly connected to the input shaft IS and always receive the torque of the input shaft IS.

The dog clutch sleeve DCS is splined to an exterior circumferential portion of the dog clutch hub DCH so as to be slidable in an axial direction, and moves in the axial direction by an actuator (not shown).

The first and second dog clutch gears DCG1 and DCG2 are directly connected respectively to the first and second transfer gears TF1 and TF2. The first and second dog clutch gears DCG1 and DCG2 are selectively engaged with the dog clutch sleeve DCS and selectively receive the torque of the input shaft IS.

Since the dog clutch DC is well known to a person of an ordinary skill in the art, detailed description thereof will be omitted.

FIG. 2A illustrates the dog clutch DC at a neutral state. As shown in FIG. 2A, the dog clutch sleeve DCS is positioned at the center and is not engaged to neither of the first and second dog clutch gears DCG1 and DCG2 at the neutral state.

FIG. 2B illustrates that the torque of the input shaft IS is transmitted to the first transfer gear TF1. As shown in FIG. 2B, if the dog clutch sleeve DCS moves to the left in the drawings by the actuator, the dog clutch hub DCH and the first dog clutch gear DCG1 are operably connected.

In this case, the torque of the input shaft IS is transmitted to the first transfer gear TF1 through the dog clutch DC.

FIG. 2C illustrates that the torque of the input shaft IS is transmitted to the second transfer gear TF2. As shown in FIG. 2C, if the dog clutch sleeve DCS moves to the right in the drawings by the actuator, the dog clutch hub DCH and the second dog clutch gear DCG2 are operably connected.

In this case, the torque of the input shaft IS is transmitted to the second transfer gear TF2 through the dog clutch DC.

FIG. 3 is an operational chart of friction elements at each shift-speed applied to a planetary gear train according to an exemplary embodiment of the present invention.

Referring to FIG. 3, two frictional elements or one frictional element and the dog clutch DC are operated at each shift-speed according to the exemplary embodiment of the present invention.

The first clutch C1 and the first brake B1 are operated and the dog clutch DC stays at the neutral state at a first forward speed 1ST.

The first clutch C1 and the second brake 132 are operated and the dog clutch DC stays at the neutral state at a second forward speed 2ND.

The first clutch C1 is operated and the dog clutch DC is operably connected to the first transfer gear TF1 at a third forward speed 3RD.

The first clutch C1 is operated and the dog clutch DC is operably connected to the second transfer gear TF2 at a fourth forward speed 4TH.

The first clutch C1 and the second clutch C2 are operated and the dog clutch DC stays at the neutral state at a fifth forward speed 5TH.

The second clutch C2 is operated and the dog clutch DC is operably connected to the second transfer gear TF2 at a sixth forward speed 6TH.

The second clutch C2 is operated and the dog clutch DC is operably connected to the first transfer gear TF1 at a seventh forward speed 7TH.

The second clutch C2 and the second brake B2 are operated and the dog clutch DC stays at the neutral state at an eighth forward speed 8TH.

The first brake B1 is operated and the dog clutch DC is operably connected to the second transfer gear TF2 at a reverse speed REV.

FIG. 4 is a lever diagram of a planetary gear train according to the exemplary embodiment of the present invention and illustrates shift processes of the planetary gear train according to the exemplary embodiment of the present invention by lever analysis method.

Referring to FIG. 4, four vertical lines of the planetary gear set PG are set as the first, second, third, and fourth rotation shafts N1, N2, N3, and N4 from the left to the right. In addition, a lower horizontal line represents a rotation speed of “0”, an upper portion of the horizontal line represents positive rotation speed, and a lower portion of the horizontal line represents negative rotation speed.

At this time, although rotation speed input to each rotation shaft is negative rotation speed, for better comprehension and ease of description, it is illustrated that positive rotation speed is input to each rotation shaft.

Hereinafter, referring to FIG. 3 and FIG. 4, the shift processes of the planetary gear train according to an exemplary embodiment of the present invention will be described in detail.

First Forward Speed

Referring to FIG. 3, the first clutch C1 and the first brake B1 are operated and the dog clutch DC stays at the neutral state at the first forward speed 1ST.

As shown in FIG. 4, since the dog clutch DC stays at the neutral state, the torque of the input shaft IS is not input to the fourth rotation shaft N4 and is input to the first rotation shaft N1 through the third transfer gear TF3 by operation of the first clutch C1. In addition, the third rotation shaft N3 is stopped by operation of the first brake B1.

Therefore, the rotation shafts form a first shift line SP1 and D1 is output through the second rotation shaft N2 that is the output element.

Second Forward Speed

The first brake B1 that was operated at the first forward speed 1ST is released and the second brake B2 is operated at the second forward speed 2ND.

In this case, since the dog clutch DC stays at the neutral state, the torque of the input shaft IS is not input to the fourth rotation shaft N4 and is input to the first rotation shaft N1 through the third transfer gear TF3 by operation of the first clutch C1. In addition, the fourth rotation shaft N4 is stopped by operation of the second brake B2.

Therefore, the rotation shafts form a second shift line SP2 and D2 is output through the second rotation shaft N2 that is the output element.

Third Forward Speed

The second brake B2 that was operated at the second forward speed 2ND is released and the dog clutch DC is operably connected to the first transfer gear TF1 at the third forward speed 3RD.

In this case, the torque of the input shaft IS is input to the first rotation shaft N1 through the third transfer gear TF3 by operation of the first clutch C1 and is input to the fourth rotation shaft N4 through the first transfer gear TF1.

Therefore, the rotation shafts form a third shift line SP3 by the gear ratios of the first transfer gear TF1 and the third transfer gear TF3, and D3 is

Fourth Forward Speed

If the dog clutch DC is released from the first transfer gear TF1 and is operably connected to the second transfer gear TF2 at the third forward speed 3RD, the fourth forward speed 4TH is achieved.

In this case, the torque of the input shaft IS is input to the first rotation shaft N1 through the third transfer gear TF3 by operation of the first clutch C1 and is input to the fourth rotation shaft N4 through the second transfer gear TF2.

Therefore, the rotation shafts form a fourth shift line SP4 by the gear ratios of the second transfer gear TF2 and the third transfer gear TF3, and D4 is output through the second rotation shaft N2 that is the output element.

Fifth Forward Speed

If the dog clutch DC returns to the neutral state and the second clutch C2 is operated at the fourth forward speed 4TH, the fifth forward speed 5TH is achieved.

In this case, the torque of the input shaft IS is input to the first rotation shaft N1 and the third rotation shaft N3 through the third transfer gear TF3 by operation of the first clutch C1 and the second clutch C2.

Therefore, the planetary gear set PG becomes a direct-coupling state, the rotation shafts form a fifth shift line SPS, and D5 is output through the second rotation shaft N2 that is the output element.

Sixth Forward Speed

The first clutch C1 that was operated at the fifth forward speed 5TH is released and the dog clutch DC is operably connected to the second transfer gear TF2 at the sixth forward speed 6TH.

In this case, the torque of the input shaft IS is input to the third rotation shaft N3 through the third transfer gear TF3 by operation of the second clutch C2 and is input to the fourth rotation shaft N4 through the second transfer gear TF2.

Therefore, the rotation shafts form a sixth shift line SP6 by the gear ratios of the second transfer gear TF2 and the third transfer gear TF3, and D6 is output through the second rotation shaft N2 that is the output element.

Seventh Forward Speed

If the dog clutch DC is released from the second transfer gear TF2 and is operably connected to the first transfer gear TF1 at the sixth forward speed 6TH, the seventh forward speed 7TH is achieved.

In this case, the torque of the input shaft IS is input to the third rotation shaft N3 through the third transfer gear TF3 by operation of the second clutch C2 and is input to the fourth rotation shaft N4 through the first transfer gear TF1.

Therefore, the rotation shafts form a seventh shift line SP7 by the gear ratios of the first transfer gear TF1 and the third transfer gear TF3, and D7 is output through the second rotation shaft N2 that is the output element.

Eight Forward Speed

If the dog clutch DC returns to the neutral state and the second brake B2 is operated at the seventh forward speed 7TH, the eighth forward speed 8TH is achieved.

In this case, the torque of the input shaft IS is input to the third rotation shaft N3 through the third transfer gear TF3 by operation of the second clutch C2, and the fourth rotation shaft N4 is stopped by operation of the second brake B2.

Therefore, the rotation shafts form an eighth shift line SP8 and D8 is output through the second rotation shaft N2 that is the output element.

Reverse Speed

The first brake B1 is operated and the dog clutch DC is operably connected to the second transfer gear TF2 at the reverse speed REV.

In this case, the torque of the input shaft IS is input to the fourth rotation shaft N4 through the second transfer gear TF2, and the third rotation shaft N3 is stopped by operation of the first brake B1.

Therefore, the rotation shafts form a reverse shift line RS and REV is output through the second rotation shaft N2 that is the output element.

The planetary gear train according to the exemplary embodiment of the present invention can achieve eight forward speeds and one reverse speed by combining the planetary gear set PG being the compound planetary gear set with three transfer gears TF1, TF2, and TF3 being the externally-meshed gears, four frictional elements C1, C2, B1, and B2, and the dog clutch DC.

Since the first, second, and third transfer gears TF1, TF2, and TF3 and the dog clutch DC are used, at least one planetary gear set and two frictional elements may be removed, compared with the planetary gear train. Therefore, manufacturing cost may be curtained by reducing the number of components.

In addition, optimum gear ratios may be set due to ease of changing gear ratios by using the first, second, and third transfer gears TF1, TF2, and TF3 as well as the planetary gear set PG. Since the gear ratios can be changed according to target performance, starting performance, power delivery performance and fuel economy may be improved.

In addition, two frictional elements or one frictional element and the dog clutch are operated at each shift-speed, and one frictional element is released and another friction element is operated or only the dog clutch changes its operating state so as to shift to a neighboring shift-speed. Therefore, shift control condition is fully satisfied.

For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “inner” and “outer” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. 

What is claimed is:
 1. A planetary gear train of an automatic transmission for a vehicle comprising: an input shaft receiving torque; an output shaft disposed in parallel with and apart from the input shaft; a planetary gear set including a first sun gear engaged with a long pinion, a second sun gear engaged with a short pinion, a common planet carrier rotatably supporting the short pinion and the long pinion, and a common ring gear engaged with the long pinion as rotation elements thereof; a first rotation shaft directly connected to the first sun gear and selectively connected to the input shaft; a second rotation shaft directly connected to the common planet carrier and directly connected to the output shaft; a third rotation shaft directly connected to the common ring gear and selectively connected to the input shaft or to a transmission housing; and a fourth rotation shaft directly connected to the second sun gear and selectively connected to the input shaft through first and second paths or to the transmission housing.
 2. The planetary gear train of claim 1, further comprising three transfer gears disposed respectively between the input shaft and the first and third rotation shafts and on the first and second paths.
 3. The planetary gear train of claim 1, further comprising a dog clutch selectively transmitting torque of the input shaft to any one of the first or second paths or not transmitting torque to either of the first or second paths.
 4. The planetary gear train of claim 2, wherein the three transfer gears comprise: a first transfer gear disposed on the first path; a second transfer gear disposed on the second path; and a third transfer gear disposed between the input shaft and the first and third rotation shafts.
 5. The planetary gear train of claim 2, wherein gear ratios of the three transfer gears are different from each other.
 6. The planetary gear train of claim 4, further comprising: a first clutch disposed between the third transfer gear and the first rotation shaft; a second clutch disposed between the third transfer gear and the third rotation shaft; a first brake disposed between the third rotation shaft and the transmission housing; and a second brake disposed between the fourth rotation shaft and the transmission housing.
 7. The planetary gear train of claim 6, wherein the first and second transfer gears and the first and second brakes are disposed at a first side of the planetary gear set, and the first and second clutches and the third transfer gear are disposed at a second side of the planetary gear set.
 8. A planetary gear train of an automatic transmission for a vehicle comprising: an input shaft receiving torque; an output shaft disposed in parallel with and apart from the input shaft; a planetary gear set including a first sun gear engaged with a long pinion, a second sun gear engaged with a short pinion, a common planet carrier rotatably supporting the short pinion and the long pinion, and a common ring gear engaged with the long pinion as rotation elements thereof; a first rotation shaft directly connected to the first sun gear and selectively connected to the input shaft; a second rotation shaft directly connected to the common planet carrier and directly connected to the output shaft; a third rotation shaft directly connected to the common ring gear and selectively connected to the input shaft or to a transmission housing; a fourth rotation shaft directly connected to the second sun gear and selectively connected to the input shaft through first and second paths or to the transmission housing; and a dog clutch selectively connecting the input shaft to the fourth rotation shaft through the first and second paths.
 9. The planetary gear train of claim 8, further comprising: a first transfer gear disposed on the first path; a second transfer gear disposed on the second path; and a third transfer gear disposed between the input shaft and the first and third rotation shafts.
 10. The planetary gear train of claim 9, wherein gear ratios of the three transfer gears are different from each other.
 11. The planetary gear train of claim 9, further comprising: a first clutch disposed between the third transfer gear and the first rotation shaft; a second clutch disposed between the third transfer gear and the third rotation shaft; a first brake disposed between the third rotation shaft and the transmission housing; and a second brake disposed between the fourth rotation shaft and the transmission housing.
 12. The planetary gear train of claim 11, wherein the first and second transfer gears and the first and second brakes are disposed at one side of the planetary gear set, and the first and second clutches and the third transfer gear are disposed at the other side of the planetary gear set.
 13. The planetary gear train of claim 8, wherein the dog clutch is adapted to selectively transmit torque of the input shaft to any one of the first or second paths or not to transmit torque to either of the first or second paths. 